He flicked the ash from his cigarette and resumed:

"Without mentioning names, the experts consulted in the other cases which—according to the late Gaston Max—were victims of 'The Scorpion,' do not seem to have justified their titles. I am arranging that you shall be present at the autopsy upon the body of Gaston Max. And now, permit me to ask you a question: are you acquainted with any poison which would produce the symptoms noted in the case of Sir Frank Narcombe, for instance?"

Stuart shook his head slowly.

"All that I know of the case," he said, "is that he was taken suddenly ill in the foyer of a West-End theatre, immediately removed to his house in Half Moon Street, and died shortly afterward. Can you give me copies of the specialists' reports and other particulars? I may then be able to form an opinion."

"I will get them for you," replied the Commissioner, the exact nature of whose theory was by no means evident to Stuart. He opened a drawer. "I have here," he continued, "the piece of cardboard and the envelope left with you by the missing cab-man. Do you think there is any possibility of invisible writing?"

"None," said Stuart confidently. "I have tested in three or four places as you will see by the spots, but my experiments will in no way interfere with those which no doubt your own people will want to make. I have also submitted both surfaces to a microscopic examination. I am prepared to state definitely that there is no writing upon the cardboard, and except for the number, 30, none upon the envelope."

"It is only reasonable to suppose," continued the Commissioner, "that the telephone message which led Inspector Dunbar to leave your house last night was originated by that unseen i